Controller and control system

ABSTRACT

A vacuum responsive controller and control system is disclosed for safely controlling the water level in paint spray booths of the type employing a high velocity air-current to entrain water from a reservoir and thereby aid in removing air-borne paint particles. The controller and system sample and signal the vacuum in the booth as an index of the dynamic conditions of water level and air velocity, and for vacuums below a threshold value, automatically raise the reservoir water level to achieve proper entrainment. The controller automatically responds to a lower vacuum to not supply water. Thus a failure of the air impeller will not result in water flowing to the reservoir. The controller includes a diaphragm moving in response to the pressure difference between vacuum and atmospheric inlets and valve means responsive to the diaphragm&#39;&#39;s flexing over a range to not feed, feed, and again not feed an air pressure signal to a command signal output which controls a water supply valve. Adjustable mechanical biasing on the diaphragm allows for changing the operation point of the controller.

United States Patent [191 Scotchmur 1 CONTROLLER AND CONTROL SYSTEM [75]Inventor: Ronald R. S c0tchmur, Schiller Park,

Primary Examiner-Tim R. Miles Assistant Examiner-Steven H. MarkowitzAttorney-Gary, Parker, Juettner, Pigott Cullinan [57] ABSTRACT A vacuumresponsive controller and control system is 11 3,738,627 2 June 12, 1973disclosed for safely controlling the water level in paint spray boothsof the type employing a high velocity aircurrent to entrain water from areservoir and thereby aid in removing air-borne paint particles. Thecontroller and system sample and signal the vacuum in the booth as anindex of the dynamic conditions of water level and air velocity, and forvacuums below a threshold value, automatically raise the reservoir waterlevel to achieve proper entrainment. The controller automaticallyresponds to a lower vacuum to not supply water. Thus a failure of theair impeller will not result in water flowing to the reservoir. Thecontroller includes a diaphragm moving in response to the pressuredifference between vacuum and atmospheric inlets and valve meansresponsive to the diaphragm s flexing over a range to not feed, feed,and again not feed an air pressure signal to a command signal outputwhich controls a water supply valve. Adjustable mechanical biasing onthe diaphragm allows for changing the operation point of the controller.1

4 Claims, 6 Drawing Figures PATENIED Jim 1 3 SHEET 2 BF 3 Nu m Q R m \mwNm FIELD OF TI-IE INVENTION The present invention is a new and improvedcontroller and control system and is especially concerned with 1 such acontroller and system for safely regulating the BACKGROUND OF THEINVENTION One type of paint spray booth employs liquid, normally water,entrained by an air current to aid in the removal of paint particlesfrom the air. This type of paint spray booth eliminates the need forwater pumps and spray nozzles and provides for relative economy andsimplicity of construction and operation. Consequently such booths haveproved popular and have met with significant commercial success.

One such paint spray booth is the presently commercially available NoPump Spray Booth manufac tured by Binks Manufacturing Company ofChicago, 111., the assignee of the present invention.

Such booths remove paint particles by drawing air, by means of animpeller, from the spraying area into a scrubbing or washing chamber.This air is drawn along a path which travels over the surface of waterheld in a reservoir, The air path and water reservoir are so arrangedthat for certainra tes of flow and water levels, the water is entrainedby the air and borne into the washing chamber where, by centrifugalaction, the water together with the greater part of the air-borne paintparticles, are precipitated out to return to the res ervoir. In thereservoir, the heavier paint particles settle to the bottom much as slitin a backwater of a stream. With the addition of chemical additives tothe water the lighter paint particles may also be broken down and causedto settle in the bottom of the reservoir. Periodically, when asufficient deposit has developed thereservoir is drained and thedeposits removed.

A proper water level and air flow rate to cause and maintain entrainmentof water and establish an efficient scrubbing or washing action iscritical to the successful operation of this type of spray booth. It isan improvement in the control system and the water level controller thatthe present invention is directed.

Prior control systems employed an air pressure or relative vacuumsampling device within the washing chamber to detect air flow and inresponse to vacuums below a certain level to cause additional water toflow from an outside source into the reservoir, thus raising its level.These control systems generally involved a diaphragm for sensing thedegree of vacuum, a double acting valve operated by the diaphragm forcontrolling a relatively high pressure air line, and an air operatedvalve for controlling the addition of water to the reservoir; the doublevalve opening the air line over a predetermined range of vacuumpressures'sensed by the diaphragm to cause the water valve to open andclosing at pressures above and below said range whereby to discontinuethe flow of water when the reservoir was filled to the proper level andalso when the spray booth was shut down or the air flow therethroughdiscontinued.

These controls have suffered several disadvantages consequent upon theinherent smallness, of the high pressure air valve, the necessity forthe same valve to be double acting and the high pressure air system ingeneral.

SUMMARY OF THE INVENTION To overcome the drawbacks of prior controllersand control systems and to provide for a simpler more easilymanufactured and operated controller, the present invention provides animprovementin sueh a system including a diaphragm operated interfacevalve for controlling the water supply at very lowai r pressure and atthe same time isolating the water arid air sys tems, and an improvedvalve structure operahle response to the vacuum pressure in the washingchamber for controlling the supply of air at very pressure to saidinterface valve, and thus controlling the of liquid to the reservoir;the improved control valve being highly sensitive to the vacuum signalto cause liquid to be added to the reservoir for vacuums range of anintermediate valve value but not for vacuurns above or below that value.x i i BRIEF DESCRIPTION OF THE DRAWINGS troller of FIG. 1 illustratingthe operation and inter relationship of its various important partsunderdiffep ent operating conditions; and i FIG. 5 is an enlarged sideview, largely in section, of

the controller and its associated parts; and i I FIG. 6 is an enlargedvertical section of the interface valve used in combination with saidcontroller.

DETAILED DESCRIPTION Referring to FIG. 1 there is depicted a spray boothgenerally designated 10. The booth 10 includes a base 1 1 which definesa liquid reservoir 12. As water is conventionally used to fill thereservoir we will here refer to the liquid as water. It should beunderstood that other liquids may be employed. The reservoir 12 has anominal or normal water level 13 and includes a stand pipe 14 extendingto that level to drain overflow water from the receiver 12.

Over the back portion of the base 11 and reservoir i2 is a verticalwashing chamber 15 having a front wall 16 which extends to just abovethe level 13. The front wall i 16 and liquid level define a slit opening1'1 which extends for the width of the chamber 15. Through slit 17 airis drawn into the chamber 15 during operation of the booth. The bottomedge of the wall preferrably serrated to aid in entraining of water. i Ii i The chamber 15 has a number of baffles posiwater and precipitatefrom the baffles 20, back wall, and the rearward portion of the sidewalls of the chamber. This collected matter is returned via a conduit 30to a trough 32 in the reservoir 12 below the level 13. The trough 32 isan inverted channel which serves to aid in recirculating the water andlighter particles while allowing heavier precipitated particles tosettle to the bottom of the reservoir.

To aid in entraining of water, a curved distribution plate 34 isprovided that extends the width of the reservoir 12. The plate 34 hasits approximately horizontal forward edge and forward portion under thelevel 13 and below the serrated bottom edge .of the wall 16 while itsrearward approximately vertical portion is affixed to the front edge ofthe collection pan 28.

The booth 10, as thus far described is known in the art and generallycorresponds to the commercially available NPB series of boothsmanufactured by the Binks Manufacturing Company of Chicago, lll. anddescribed, for example, in their Bulletin No. A27-20R.

In overall operation of the booth 10, air is drawn from the paintspraying area in front of the wall 16 (which area is usually covered bya hood extending outward from the sides and top of the booth through theopening slit 17 into the chamber 15, and thence by and about the baffles20 and out through the impeller 22 and duct 26. In the process water isentrained by the high velocity air at the opening 17. This air-bornewater strikes the various baffles 20 which cause a series of rapidchanges of direction. These rapid changes of direction separate thewater and air mixture due to cen- 'rifugal action. The water and paintparticles are thus precipitated to the collection pan 28 and to thereservoir 12. The precipitated paint is collected in the reservoir foreventual removal.

As mentioned before, the size of the slit opening 17 is critical forproper entrainment and operation of the booth 10. Obviously duringoperation, some water is air-borne and flowing in the chamber andcollection pan. Thus the reservoir water level will drop as the booth isinitially put into operation. Also during operation water is lostthrough evaporation into the exhausted air. The rate of such loss willvary from time to time depending upon the air flow rate and the amountof water entrained and the relative humidity of the air drawn from thespraying area. Thus the water level will not of itself remain constantand must be controlled and additional water must be added to make up forlost water.

It is in accordance with the present invention that the booth 10 isequipped with an improved liquid level control system, generallydesignated 50. This system 50 includes means 56 for sampling the vacuumin the washing chamber 15, a new and improved controller 70, coupledthereto by a vacuum line 51, and means for supplying liquid to thereservoir. This liquid supply means includes an interface valve 54 whichis connected to a liquid source 58 and to a reservoir liquid supply line60, and which is coupled to the controller for control thereby over acontrol signal air line 55.

The signal transmitted to the controller 70 via the line 51 is thevacuum level present at the sampling means 56. The means 56 ispreferably a fixture mounted through the backwall and into the upperpart of the chamber 15. This vacuum signal is related to air flow, whichflow is dependent upon the size of the opening 17 and thus on the actualwater level in the reservoir 12. A constant air pressure source 52, suchas a conventional regulator-filter, is connected to the controller andcoupled via a line 57 to an air pressure source such as the air linenormally available in a paint spraying shop.

The interface valve 54 operates on a low pressure air signal from line55 and controls passage of water from the supply pipe 58 to thereservoir filling hose 61. Also, as will be later described, the valveisolates the air and control systems from the water system. The valve 54bypasses a fill valve 59, which is used to initially fill the reservoir12 and is thereafter kept closed during operation of the system.

The general principles of operation, although not the detailedconstruction, of the controller 70 can be best understood by referenceto FIGS. 24, where the simplified controller constructed in accordancewith principles of the invention is generally designated 70.

Referring to those figures it can be seen that the controller 70'includes a two-part housing 71 and a flexible diaphragm 72 clampedbetween the housing parts 71 and dividing the housing into an upper zone73 and lower zone 74. The diaphragm 72 preferably comprises a thinflexible membrane and a pair of disc supports 75 which sandwich thecentral part of the membrane therebetween. The lower disc 75 serves toreceive one end of a spring 76 which biases the diaphragm toward theupper zone 73. The bottom of the spring 76 is seated in a screwadjustment 77. The'adj'ustment 77 constitutes means for adjusting theoperation point of the controller 70.

The lower zone 74 is in communication with the vacuum signal line 51through an inlet 51' while the lower portion 73L of the upper zone 73communicates via a port 78 to atmosphere. The port 78 and inlet 51'serve as a pair of fluid pressure inlets. Thus for a setting of thescrew adjustment 77 the position of the diaphragm is governed by therelative pressure between the inlets 51, 78.

The upper portion 73U of the upper zone 73 is in communication via anair flow restriction 79, such as a small opening, to the regulated lowpressure input 53, and via an outlet 55 to the interface valve line 55.Two pressure paths are defined by the housing from the opening 79, oneto the command signal outlet 55' and the other to zone 73L, whichbecause of its opening 78 serves as a air pressure vent.

Valve means is positioned in the zone 73 for governing the transfer of apressure signal from the orifice 79 to the interface control valve line55 in response to the position of the diaphragm 72. The valve means 80serves to shut or open the second air pressure path to the vent zone73L. The valve means 80 includes a valve stem 81 connected with theupper diaphragm disc 75 to travel with the diaphragm 72 and having afirst disc shaped valve member 82 affixed to the top of the stem 81.

The valve member 82, sits against a second valve member 84, as shown inFIGS. 2 and 3. The member 84 is generally of washer shape and looselymounted about the stem 81 below the disc member 82. The valve member 84has a planar upper surface and is biased toward the disc member 83 by aspring 86. The spring 86 is preferably seated against a base 87 restingon the stem 81. the second valve member 84 is so arranged as to be ableto be seated against a flange 85 of the housing 71, as in the positionshown in FIGS. 3 and 4.

In overall operation, the controller 70'. functions to operate theinterface valve by sending a signal over the line 55 for a communicatedvacuum signal via the line 51. The operating point of the controller 70'is adjusted so that this threshold vacuum signal represents too low of awater level and improper entrainment of water by the booth. Thecontroller 70' functions to not communicate such an air pressure controlsignal over line 51 in response to a proper vacuum or, in accordancewith one feature of the invention, in response to the substantial lackof any vacuum.

The normal operating state, with sufficiently high water level isdepicted in FIG. 2. Because of the relatively high vacuum communicatedvia line 51 the diaphragm 72 is held down. This has moved down the valvestem 81 and the two valve members 82, 84 to thus keepthe valve member 84away from its valve seat. Under these conditions air fromthe constantpressure source 53 is bled off out of the chamber 73U through the valveopening to the area 73L and thus to the atmosphere via the opening 78.This air flow is indicated by the-arrows in FIG. 2.

The valve opening and the vent 78 are larger than the orifice 79 andthus insure that the line 55 will be at approximately atmosphereicpressure under the conditions of FIG. 2.

When the water level drops in the reservoir, the vac uum communicated byline 51 decreases (the air pressure rises in the wash chamber). Thespring 76 thereupon urges the diaphragm upward which causes the stem 81and its two valve members 82 and 84 to rise. For a sufficiently smalldrop in vacuum, these members rise enough to seat the valve member 84and to close off the air path to the zone 73 and the atmosphere. Thisstate is depicted in FIG. 3.

The pressure in the zone 73U rises as air flows through the opening 79and results in a rise in pressure in line 55 to operate the interfacevalve 54, and supply more water to the reservoir 12.

When, in normal operation, the water level raises again to the properlevel, the vacuum in the chamber 15 increases. This increase in vacuumis communicated over the line 51 and causes diaphragm 72 and the valvemember 84 to be pulled downward reopening the passageway to the zone 73Land opening 78 to atmospheric pressure.

This drops the pressure in the zone 73U which pressure drop (via outlet55 and line 55) is communicated to the interface valve 54 to shutoff theflow of water to the reservoir.

If, however, some abnormal operating conditions should occur, such as aninterruption of electrical power to the impeller motor 24, the fall invacuum in the washing chamber 15 would be drastic and the controllerwould assume the configuration of FIG. 4. That is, the lack of vacuumwould allow the spring 76 to drive the diaphragm 72 and valve stem 81including the valve member 82 upward. The spring 86, which is of a lowerspring constant than the spring 76, would initially maintain the washershaped valve member 84 seated against the bottom of the member 82.However, as the member 84 reached the lip 85 it would stop its upwardmovement and as the diaphragm 72 continued to rise, the members 82 and84 would move apart reopening a bleed path from the zone 73U to the zone73L and out the opening 78. This would again prevent the rise ofpressure in the zone 73U and prevent the activation of the interfacevalve 54.

Having described the general principle of operation of the controller70, by means of the simplified controller depicted in FIGS. 2-4, we cannow return to consider the preferred construction thereof.

Referring now to FIG. 5 there is depicted the controller 70 of FIG. 1,together with an air filter and pressure regulator 52 andinterconnecting couplings. A shop air pressure line of, for example,20-200 psig is connected to the input 57 of the regulator 52. Theregulator 52 serves to produce a regulated low pressure output at, forexample, 0.7 to 1.0 psig. This output is coupled, via the line 53, to aninput coupling 53' to housing 71 of the controller 70.

The housing 71 of the controller 70 is formed of four major components:a lower casing 71L, an upper casing 711], a projecting body 718 andretainer 71R mounted therein. The lower casing 71L is shaped generallylike a wide wine glass with a central depending stem and with horizontalextending flanges 100. The upper casing 71U is shaped like a shallowdish which covers the lower casing 71L and has a flange 101 sized toconform to the flange thereof. A gasket and the circular margin portionof the diaphragm 72 is sandwiched between the two flanges 100, 101 andthe casing 71L, 71U fastened together and to a mounting bracket 102 bymeans of a plurality of bolts and nuts 106.

The body 718 which is generally ofa hollow cylinder shape is affixed bymeans of machine screws 107 to the casing 71U in a sealed relationthereto. The body 718 is affixed about a central opening in the casing711]. The retainer 7 1R is a solid cylinder having a central large boreforming the zone 73U and is threadably mounted into the top of the body718. A plurality of horizontal openings 111 communicate air pressure toan annular zone 112 formed about the retainer 71R between it and thebody 71B and from which a fixture 55' allows the command signal to befed to the line 55. The constant air pressure of line 53 is coupledthrough a connector 53 to the orifice 79 and then to the zone 73U bymeans of a small bore 116.

The lower casing 70L includes a threaded inlet 51' into which an elbowfixtureSlE is fitted and to which the line 51 is coupled. An additionalopening from the chamber 74 is provided via a depending wall extension119 which threadably receives a vacuum gauge 120.

The gauge gives a visual reading of the vacuum in the washing chamber 15and aids in the set-up, calibration and supervision of the controlsystem 50 and controller 70.

The adjustment 77 consists of a screw 91 threadably mounted at 92 withinthe stem 90 and having a slotted head 93 which is sized to closely fitwithin a bore 94 extending upward from the bottom of the stem 90. Thehead 93 has an encircling groove 95 in which an O-ring seal 96 ismounted. The seal 96 insures that the chamber 74 remains vacuum-tightdespite the externally adjustable screw 91. A plug 97 is threadablymounted into the bore 94 at the bottom of the stem 90, as furtherprotection. The point of the screw 91 sits into a button 99 whichreceivesthe spring 76.

The diaphragm 72 is similar to that of the diaphragm 72 of FIGS. 2-4except that its flexible marginal portion was exaggerated in thosefigures to make its operation clearer. In fact, the travel of thediaphragm and its range of positions is small. However, it is sufficientto operate the valve means 80 which is generally similar inconstruction; the main difference being that the bottom of the valvemember 84 is formed with a spring receiving hollow and an O-ring seal120 is provided between the members 82, 84.

The provision of the O-ring 120 serves to insure a good seal and providefor a small range of closed positions about the critical position forclosing the valve means 80.

It should be noted that the spring 86 is compressed as the diaphragm 72flexes upward beyond the position in which the valve member 84 makescontact with the holder surface 85 and that the force of thiscompression is added to the air pressure force on the upper surface ofthe diaphragm 75 to tend to close the valve means 80. It should also benoticed that because of the smaller circumference of the disc member 82that the bleed opening formed by an upward movement thereby is smallerthan the corresponding opening formed by a similar increment of movementdownward by the member 84. The net result of the resiliency of theO-ring 120, the relative small size of disc member 82 and the addeddownward force of the spring 86 is to provide a small range of diaphragmpositions for which the bleed path is effectively closed rather thanjust a single position, whereby to provide for adequate supply of waterso that the valve is not constantly turning off and on.

When the valve members 82 and 84 are closed, air is supplied at very lowpressure, e.g., 1 psig. or less, via the line 55 to the interface valve54. As shown in FIG. 6, this air is supplied to the upper side of alarge diameter diaphragm 110 which is sealingly clamped between the twohalves ofa shell or housing 111. The upper half of the housing togetherwith the air inlet 55 and the diaphragm 110 constitutes a sealedchamber, whereby the air system of the controller is isolated from thewater system. The lower half of the housing is suitably provided withone or more breather ports 112, whereby to maintain the lower side ofthe diaphragm at atmospheric pressure, and also to accommodate drainageof water therefrom should a leak develop in the valve stem seal 116,which is about to be described.

The housing 111 is attached centrally of the bottom wall thereof to theupper end of a valve body 113, and a valve stem 114 extends slidablyupward through the body into the housing 111 for engagement with andactuation by the lower one of the diaphragm reinforcing or supportingdiscs 115. A seal assembly 116 is provided within the body 113 about thestern 114, but even if this seal should develop a leak the air controlsystem is still protected from the water system by the diaphragm 110.

The valve body 113 is of three part construction including a centralfluid section 117 provided with a water inlet 118, a water outlet 119and an intervening valve port and seat 120 which are preferably coaxialwith the valve stem; a lower valve section 121 housing a valve 122and avalve spring 123 which normally biases the valve to closed position onthe seat 120; and an upper actuator section 124 housing a valve actuator125 which engages the valve stem 114 for actuation thereby and which iscoupled to the valve 122 by an intervening pin 126 that parallels but isoffset from the valve port. As shown, the valve 122 is preferablycomprised ofa generally annular holder 127 with which the pin or shaft126 is integrally formed, and a resilient seating member 128 detachablymounted on the holder.

By virtue of the described structure, including the detachable,preferably screw-threaded, assembly of the body parts, the valve section121 can easily be removed from the fluid section 117 for repair of thevalve and replacement of its sealing member 128. Similarly, easy accessis had to the sealing assembly 116 by removal of the upper or actuatorsection 124. Relative to the latter, it is to be further noted that thesmall diameter of the valve stem or pin 114 aids in minimizing thepossibility of water leakage. Considering the large area of thediaphragm 110, which is preferably in the order of 8 to 10 square inchesor more, and my preference to have the valve open in the direction ofwater flow, only a very low air pressure is required on the diaphragm tooperate the valve. At the same time, when either of the controllervalves 82 or 84 is opened, the upper side of diaphragm is immediatelyvented to atmosphere and pressure on the opposite sides thereofequalized, whereby the spring 123 can instantly close the valve 122 andstop the flow of water to the reservoir. Thus, the overall controlsystem is very accurate and highly reliable.

From the above description it should be clear that an improvedcontroller and control system has been described which provides not onlyfor adding of liquid to the reservoir in response to vacuum below athreshold level, but protects against continued supply of water when theair impeller fails. The described system and controller are easy andeconomical to manufacture, install and operate.

While particular embodiments of the invention have been shown anddescribed, it will be obvious to those skilled in the art that changesand modifications'may be made without departing from the invention inits broader aspects and, therefore, the aim in the appended claims is tocover all such changes and modifications as fall within the true spiritand scope of the invention.

What is claimed is:

1. in a control system for an air washing booth of the type in whichimpelled air entrains liquid from a reservoir into a washing chamber andin which the liquid surface level in the reservoir must be controlledwithin narrow limits for proper entrainment, the improvement comprising:

means for sampling the vacuum in the washing chamber and deriving asignal related thereto;

means for supplying liquid tothe reservoir;

an interface valve for controlling said liquid supplyin; means andincluding operating means responsive to very low air pressure;

a low pressure air supply for said operating means;

and

a controller including valve means connected to said air supply and saidoperating means and having vent means therein for selectively connectingsaid air supply to said operating means and venting both said airsupplyand said operating means, said controller being coupled to said samplingmeans and responsive to the vacuum signal from said meansto operate saidvalve means to connect said air supply to said operating means inresponse to a signal representative of a vacuum less than a firstthreshold value, but not for a signal representative of a vacuum lessthan a second value which is less than the first value,

whereby liquid is supplied to the reservoir at vacuums intermediate saidfirst and second values to maintain a given liquid surface level in thereservoir, but not at vacuums above or below said values to preventoverfilling of the reservoir.

2. In a control system as set forth in claim 1, said interface valvecomprising a normally closed liquid controlling valve and the operatingmeans thereof comprising an air pressure responsive diaphragm ofrelatively large area for opening the valve at low air pressures, saiddiaphragm also isolating the liquid part of the system from the air partof the system.

3. In a control system as set forth in claim 2, said diaphragm beingcoupled to said liquid controlling valve by a small diameter valve stem,and sealing means for said stem, said sealing means and the smalldiameter of said stem further aiding in isolating the liquid part of thesystem from the air part thereof 4. In a control system as set forth inclaim 1, said controller valve means comprising a first valve opened atvacuums greater than said first value and seated at vac uums less thansaid first value, and a second valve normally seated on said first valveand opened at vacuums less than said second value.

1. In a control system for an air washing booth of the type in whichimpelled air entrains liquid from a reservoir into a washing chamber andin which the liquid surface level in the reservoir must be controlledwithin narrow limits for proper entrainment, the improvement comprising:means for sampling the vacuum in the washing chamber and deriving asignal related thereto; means for suppLying liquid to the reservoir; aninterface valve for controlling said liquid supplying means andincluding operating means responsive to very low air pressure; a lowpressure air supply for said operating means; and a controller includingvalve means connected to said air supply and said operating means andhaving vent means therein for selectively connecting said air supply tosaid operating means and venting both said air supply and said operatingmeans, said controller being coupled to said sampling means andresponsive to the vacuum signal from said means to operate said valvemeans to connect said air supply to said operating means in response toa signal representative of a vacuum less than a first threshold value,but not for a signal representative of a vacuum less than a second valuewhich is less than the first value, whereby liquid is supplied to thereservoir at vacuums intermediate said first and second values tomaintain a given liquid surface level in the reservoir, but not atvacuums above or below said values to prevent overfilling of thereservoir.
 2. In a control system as set forth in claim 1, saidinterface valve comprising a normally closed liquid controlling valveand the operating means thereof comprising an air pressure responsivediaphragm of relatively large area for opening the valve at low airpressures, said diaphragm also isolating the liquid part of the systemfrom the air part of the system.
 3. In a control system as set forth inclaim 2, said diaphragm being coupled to said liquid controlling valveby a small diameter valve stem, and sealing means for said stem, saidsealing means and the small diameter of said stem further aiding inisolating the liquid part of the system from the air part thereof.
 4. Ina control system as set forth in claim 1, said controller valve meanscomprising a first valve opened at vacuums greater than said first valueand seated at vacuums less than said first value, and a second valvenormally seated on said first valve and opened at vacuums less than saidsecond value.